MSM/QSD devices are known to have bad hardware design where the modem controls GPS, audio (including mic), RAM, NAND.However, some MSM/QSD SoCs don't ship with a modem embedded, so they may be good targets.

Nvidia Tegra 2 devices are known to be too slow with Replicant, since there is no NEON nor any free hwcomposer/gralloc.Perhaps newer Tegra SoCs (which include NEON) would be faster on Replicant.

Motorola devices manufactured after the Milestone (inclusive). These have non-free locked bootloaders that check kernel signatures. sbf_flash, a utility capable of flashing Motorola devices, is non-free.

Really BAD: has too much proprietary things and the hardware is bad for freedom: modem transport is over HSIC (high speed USB), but it controls the GPS (included in the modem chip), audio and perhaps NAND too

There is a small number of Do-it-yourself phones where the user is expected to assemble his phone. Examples of this include the odroid phones with comes with android support.However the issue is that not all users wants or are able to assemble their own phones.